Apple has just released iPhone 3.1 SDK Beta to developers:

iPhone SDK 3.1 beta and iPhone OS 3.1 beta are now posted to the iPhone Dev Center. These versions are for development and testing only and should be installed on devices dedicated to iPhone OS 3.1 beta software development. Please read the iPhone OS Pre-Install Advisory and the iPhone SDK 3.1 beta release notes before downloading and installing.

Available now via developer.apple.com, and TiPb once again reminds non-developers to stay clear, avoid potential problems, and wait for the general release.

(Thanks Quickpwn.com and everyone else for the tip!)